A locus within chromosome band 13ql4 has been found to be deleted in a proportion of retinoblastomas and presumably is within a few hundred thousand base pairs of the retinoblastoma locus. Using "chromosome walking" techniques, the neighboring chromosomal DNA will be isolated and a restriction map of 100 to 400 kilobases of surrounding DNA will be determined. Single-copy sequences useful as probes will be subcloned and studied for their ability to detect restriction fragement length polymorphisms. Any polymorphic sites discovered will be tested for linkage with the locus for hereditary retinoblastoma by using DNA samples (collected in earlier years of this research grant) from members of 14 families with hereditary retinoblastoma. The polymorphic loci within 13q14 will also be used to search for localized regions of homozygosity in tumors found to be heterozygous elsewhere on 13q. Finding a region of homozygosity in a set of tumors may help to define the chromosomal region including the retinoblastoma locus. Also, the single-copy probes will be tested for their ability to detect homozygous deletions in additional retinoblastoma tumors. It is hoped that a number of tumors with overlapping deletions will be indentified, and that a smallest region of overlap wll be completely mapped. Finally, the single- copy probes isolated will be tested for their ability to hybridize to mRNA derived from retinoblastomas and retinal cell lines. A transcribed sequence within 13q14 which is deleted, rearranged, or otherwise mutant in retinoblastomas would be a likely candidate for the retinoblastoma gene. The isolation of such a sequence would form the basis for additional future studies aimed at confirming the identity of such a sequence and at undestanding the mechanism of action of the retinoblastoma gene.